Special orientation and postural stability are important functions of daily life. When the sensory-motor mechanisms maintaining equilibrium are sufficiently disturbed, a fall results. The high incidence and severe consequences of falling are significant public health concerns in the elderly. We will study how the CNS utilizes visual, vestibular, and somatosensory inputs to generate behavioral strategies which are necessary to maintain postural stability, and how these strategies change with age and disease. Subjects will stand on a moveable force-detecting platform surrounded by a moveable visual scene. Angular motion of the ankles, body, head, and eyes will be measured during a variety of tasks designed to challenge postural stability. By modulating the subjects' support surface and or visual surround proportional to postural sway, we will assess the contributions of each sensory system to equilibrium control both in isolation and during imposed sensory conflicts. We will study mechanisms underlying recovery of equilibrium following specific postural disturbances (support surface, visual surround, head and eye movement), and under varied conditions of sensory context, including the effects of eye-object distance, visual field, and head and eye orientation relative to the body. We will further isolate vestibular, neck, and visual interactions influencing spacial orientation without the disturbance of postural sway. Subjects will sit in a special rotary chair within an optokinetic drum. Eye, head, and body (chair) rotation will be recorded during active and passive head movements, with the head fixed relative to the body or free to move. The vestibulo-ocular, cervico-ocular and optokinetic reflexes serving to stabilize the visual image will be investigated in isolation and during specific interactions. We will employ optical (2x lenses) manipulation of the visual frame of reference to generate adaptive modifications in the vestibular control of eye, head, and body orientation, and will assess potential degeneration of adaptive mechanisms in the elderly. Knowledge from these studies will be applied in the developmnt of prognostic and rehabilitative tools for patients suffering from disequilibrating vestibular disease.